Retinal glial cells, known as Müller cells, play an important role in maintaining the internal geometry of the retina. Müller cells are oriented radially, parallel to the light path through the retina. The outer ends of Müller cells form tight junctions with the inner segments of the photoreceptors, and these numerous connections have the appearance of a continuous layer, the outer limiting membrane (layer 3 of the retina).
Thejunctionbetweenlayers1and2oftheretinainadultsrepresentsthesurfaceofcontactbetweentheanteriorandposteriorwallsoftheembryonicopticcupduringdevelopmentandisstructurallyweak.Retinaldetachmentisseparationatthissurfaceandcancauselossofvisionbecauseofdisplacementoftheretinafromthefocalplaneoftheeye.Itcanalsoleadtothedeathofphotoreceptorcells,whicharemaintainedbythebloodsupplyofthechoroid(thephotoreceptorlayeritselfisavascular).Deteriorationofthepigmentedepitheliumcanalsoresultinmaculardegeneration,acriticallossofhigh-acuitycentralandcolorvisionthatdoesnotaffectperipheralvision.
Inside the external limiting membrane is the outer nuclear layer (layer 4) that contains the cell bodies and nuclei of the rods and cones. The outer plexiform layer (layer 5) contains synapses between the photoreceptors and retinal interneurons, including bipolar cells and horizontal cells, whose cell bodies are found in the inner nuclear layer (layer 6). This layer also contains the cell bodies of other retinal interneurons (the amacrine and interplexiform cells) and the Müller cells.
The inner plexiform layer (layer 7) contains synapses between the retinal neurons of the inner nuclear layer, including the bipolar and amacrine cells, and the ganglion cells, whose cell bodies lie in the ganglion cell layer (layer 8). As previously mentioned, the ganglion cells are the output cells of the retina; it is their axons that transmit visual information to the brain. These axons form the optic fiber layer (layer 9), pass along the inner surface of the retina while avoiding the fovea, and enter the optic disc, where they leave the eye as the optic nerve. The portions of the ganglion cell axons that are in the optic fiber layer remain unmyelinated, but they become myelinated after they reach the optic disc. The lack of myelin where the
Folding of membrane •Fig. 8.3 Rods and Cones. Thedrawingsatthebottom showthegeneralfeaturesofarodandacone.Theinsets showtheoutersegments.
axons cross the retina helps permit light to pass through the inner retina with minimal distortion.
The innermost layer of the retina is the inner limiting membrane (layer 10) formed by the end-feet of Müller cells.
Structure of Photoreceptors: Rods and Cones
Each rod or cone photoreceptor cell is composed of a cell body (in layer 4), an inner and an outer segment that extend into layer 2, and a set of synaptic terminals that synapse in layer 5 onto other retinal cells (Fig. 8.3 ). The outer segments of cones are not as long as those of rods, and they contain stacks of disc membranes formed by infoldings of the plasma membrane. The outer segments of rods are longer, and they contain stacks of membrane discs that float freely in the outer segment, having completely disconnected from the plasma membrane when formed at the base. Both sets of discs are rich in visual pigment molecules, but rods have a greater visual pigment density, which partly accounts for their greater sensitivity to light. A single photon can elicit a rod response, whereas several hundred photons may be required for a cone response.